1. Field of the Invention
The invention relates generally to a print carriage for the deposition of a substance onto a substrate using printing techniques and the like. The invention further relates to a printer provided with such a print carriage and to procedures for performing deposition in a continuous process, in particular in the fields of textile printing and finishing.
2. Description of the Related Art
Systems for inkjet printing of images and text onto a substrate are generally known. Many such systems are adapted to desktop or office application and are well suited for performing printing onto A3 or A4 sized paper or the like. For wider substrates, more specialized machinery is required, in particular when high speed is important. For such applications, inkjet printing techniques may be used but lithographic and conventional printing techniques are still generally favoured.
For textiles, inkjet printing techniques have also recently been developed as an alternative to traditional printing, dyeing and coating techniques. These techniques are generally distinct from those used in the graphics field, due to material and dyestuff considerations. Attempts have also been made to adapt inkjet deposition techniques for textile upgrading and finishing procedures. A characteristic of these processes is often that they require considerable volumes of product to be deposited across the whole textile surface. In many situations, the uniformity of the deposition or coating is of paramount importance as the quality of the fabric depends upon it. This uniformity may be important from a visual perspective (absence of streaks or blemishes) and also from a functional perspective (waterproofing or flame retardancy).
There are currently two main system configurations used for inkjet printing: fixed array systems and scan and step arrangements. Both are mainly used with drop on demand (DoD) techniques but may also be used with continuous inkjet (CIJ) techniques.
Fixed array systems allow printing of a continuously moving substrate at relatively high production speeds. A fixed array of print heads is arranged across the width of the substrate and the nozzles are activated to deposit material as required onto the substrate which is in continuous motion below the print head array. Typically fixed array systems are used for narrow width substrates on continuous reel to reel web systems, as only a few print heads are required to cover the width of the substrate. The use of fixed array inkjet procedures for textile finishing is described in European Patent EP-B-1573109.
Fixed array systems have a number of drawbacks, mainly related to the low flexibility and lack of redundancy in such a printing system. When printing onto a wide substrate with a fixed array system, a large number of print heads are required to straddle the width of the substrate, leading to a high capital cost for the printing system. If the required substrate speed is below the maximum speed of the print head (e.g. due to other slower processes), then this extra system capacity cannot be usefully exploited and is wasted i.e. at anything below maximum speed, the printing system is making inefficient use of the print heads present. The resolution across the substrate width is fixed by the position of the print head nozzles and cannot therefore be readily varied. When maintenance of a print head is required, the substrate must stop and the array must be moved away from the substrate to allow access to the print heads. This is often a relatively complex operation and the downtime associated therewith can be costly. In the event that a nozzle fails during printing, a single vertical line appears on the substrate, which is a particularly visible mode of failure and represents a complete 100% failure to deposit material in the localized area. Printing a continuous image also requires a complex continuous data handling system. The system must continuously feed data to the print head nozzles, to maintain the image continuously printing on the substrate and there is no obvious break point (or time) where memory can be reloaded. This means that many fixed array printing systems have a repeat length dependant on their memory capacity, after which the image is simply repeated. This situation can be avoided by using dynamic memory handling where data is fed into memory as fast as it is fed out to the print heads but this requires a significantly more complicated memory management system.
Scan and step arrangements operate to scan a print head carriage across the width of a stationary substrate to print a horizontal band or swathe. The substrate is then precisely incremented forwards, before the print head carriage makes another pass across the stationary substrate to print a second swathe. Such systems are typically used for printing onto wide substrates of up to 5 m where a fixed array would be impractical. They are also used in applications where lower productivity is acceptable i.e. wide format commercial graphic arts printing.
Scan and step systems also have a number of drawbacks, mainly focused on the low productivity and the stepping nature of the substrate motion. In particular, the stepping of the substrate means that such a system has poor compatibility when used as a component or process within a continuous production line. The time taken to increment or step the substrate cannot be used for printing and limits productivity. The stepping motion also means that the substrate must be rapidly accelerated and decelerated, which requires powerful motors and a high level of control when dealing with wide substrates on heavy rollers. The stepping motion must also occur with high accuracy and repeatability, as this motion affects the down web resolution and thus the quantity of material deposited (for functional applications) or the image quality (for imaging applications). According to one device disclosed in EP-A-0829368, one or more printheads may be oriented to scan the width of a textile web at a bias angle. By printing diagonally, the printheads may operate for longer at their maximum traverse velocity. The loss of efficiency due to acceleration and deceleration of the printhead is thereby reduced although operation still takes place in scan and step mode.
All of these drawbacks have hitherto made continuous, high-speed and highly uniform deposition onto wide substrates difficult to achieve. In particular, the reliability of print heads for such operations is still far from optimal. A DoD nozzle requires continuous preventative maintenance in order to keep it functioning correctly, which is a key element in system design. If the nozzle is not used for a period it will block and not fire when subsequently required. For scan and step systems, the scanning motion of the print heads allows the turn around time at the end of each pass to be available for regular maintenance of the print heads. This may involve the cleaning of each jet or nozzle to prevent blockage and/or spitting of ink from idle nozzles. Nevertheless, the maintenance time comes at the expense of intermittent motion of the substrate. This can be a cause of additional indexing faults and wear in the drive train. Furthermore, the rapid acceleration of the print cartridge at each traverse is a potential source of mechanical failure and a design limitation.
In an array configuration, regular maintenance opportunities are not available. There have been many attempts in the inkjet industry to compensate for missing nozzles or malfunctioning nozzles. U.S. Pat. No. 4,907,013 discloses circuitry for detecting a malfunctioning nozzle in an array of nozzles in the inkjet print head. If the printer processor is unable to compensate for the malfunctioning nozzle by stepping the print head and using non-malfunctioning nozzles during subsequent passes over the print medium, the printer is shut down. U.S. Pat. No. 4,963,882 discloses using multiple nozzles per pixel location. In one embodiment, two ink droplets of the same colour are deposited upon a single pixel location from two different nozzles during two passes of the print head. U.S. Pat. No. 5,581,284 discloses a method for identifying any failed nozzle in a full width array print bar of a multicolour printer and substituting at least one droplet from a nozzle in another print bar having a different colour of ink. U.S. Pat. No. 5,640,183 discloses a number of droplet ejecting nozzles are added to the standard column of nozzles in a nozzle array, so that a number of redundant nozzles are added at the ends of each column of nozzles. The print head is shifted regularly or pseudo-randomly such that a different set of nozzles prints over the first printed swathe during a subsequent pass of the print head in a multi-pass printing system. U.S. Pat. No. 5,587,730 discloses a thermal inkjet printing apparatus having a redundant printing capability including a primary print head and a secondary print head. In one mode, if the primary print head fails, the secondary print head prints ink drops of the first colour in place of the primary print head.
A printing device is disclosed in U.S. Pat. No. 6,439,786 that attempts to synchronise motion of a web of paper with traverse of a print head in order to achieve continuous paper feed. The print head is mounted to traverse on a beam that can be angled in two directions with respect to the feed direction. On each traverse the print head moves with the paper to produce a resultant horizontal print band on the moving paper.
In a further device disclosed in Japanese Publication JP10-315541 a serial printer is described for enhancing print resolution in the paper transport direction. This is achieved by continuously transporting the paper whereby effects of backlash in the transport mechanism may be reduced. Printing onto the moving substrate results in diagonal swathes which may be aligned with each other in single or double pass movement. The device is directed to printing onto sheets of paper and is not concerned with enhancing printing speed on large format substrates. In particular, when printing on both the forward and reverse passes, the print head addresses only unprinted areas of the paper, leading to inefficient nozzle usage. Furthermore, the document fails to address the need for enhanced head length for printing wide swathes onto large format substrates.
A recent development is described in unpublished application WO2009/056641, the contents of which are hereby incorporated in their entirety, in which a substance is deposited onto a continuous supply of substrate by traversing a deposition arrangement across the substrate to deposit the substance in a number of swathes. The substrate may be carried by a transport arrangement in the form of a conveyor belt. By synchronising the transport and traverse motions, the swathes can be made to complement one another, thus achieving substantially complete coverage of the substrate. The principle combines advantages of both scan and step and fixed array systems to achieve reliable printing with continuous substrate motion.
According to one embodiment of the device disclosed in WO2009/056641, two complementary swathes of the substance are deposited by two carriages, each mounted for independent motion on a respective beam. Each carriage comprises a plurality of heads, thus achieving a wide swathe in the transport direction and more efficient coverage. While this arrangement has been found to operate in a satisfactory manner, the setting up thereof is difficult and variations in transport speed or other print parameters can require recalibration. Any motion of the substrate with respect to the transport belt between the first and second carriages can be catastrophic to the result. The same applies to irregularities in the motion of the transport belt. These and other difficulties become more significant as the substrate width and transport speed increase.